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benhoob |
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\section{Introduction}
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warren |
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In this note we describe a search for new physics in the 2011
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benhoob |
1.1 |
opposite sign isolated dilepton sample ($ee$, $e\mu$, and $\mu\mu$).
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warren |
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The main sources of high \pt isolated dileptons at CMS are Drell Yan and \ttbar.
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benhoob |
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Here we concentrate on dileptons with invariant mass consistent
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with $Z \to ee$ and $Z \to \mu\mu$. A separate search for new physics in the non-\Z
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sample is described in~\cite{ref:GenericOS}.
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We search for new physics in the final state of \Z plus two or more jets plus missing transverse energy (MET). We reconstruct the \Z boson
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warren |
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in its decay to $e^+e^-$ or $\mu^+\mu^-$. Our search regions are defined as MET $\ge$ \signalmetl~GeV (loose signal region) and MET $\ge$ \signalmett~GeV (tight signal region), and two or more jets. We use data driven techniques to predict the
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benhoob |
1.1 |
standard model background in this search region.
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Contributions from Drell-Yan production combined with detector mis-measurements that produce fake MET are modeled via MET templates based on photon plus jets events.
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Top pair production backgrounds, as well as other backgrounds for which the lepton
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warren |
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flavors are uncorrelated such as di-bosons ($VV$) and DY$\rightarrow\tau\tau$, are modeled via $e^\pm\mu^\mp$ subtraction.
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benhoob |
1.1 |
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As leptonically decaying \Z bosons is a signature that has very little background, they provide a clean final state in which to search for new physics.
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Because new physics is expected to be connected to the Standard Model Electroweak sector, it is likely that new particles will couple to W and Z bosons.
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For example, in mSUGRA, low $M_{1/2}$ can lead to a significant branching fraction for $\chi_2^0 \rightarrow Z \chi_1^0$.
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In addition, we are motivated by the existence of dark matter to search for new physics with MET.
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Enhanced MET is a feature of many new physics scenarios, and R-parity conserving SUSY again provides a popular example. The main challenge of this search is therefore to
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understand the tail of the fake MET distribution in \Z plus jets events.
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The basic idea of the MET template method~\cite{ref:templates1}\cite{ref:templates2} is to measure the MET distribution in a control sample which has no true MET and a similar topology to the signal events.
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In our case, we choose a photon sample with two or more jets as the control sample.
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Both the control sample and signal sample consist of a well measured object (either a photon or a leptonically decaying $Z$), which recoils against a system of hadronic jets.
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In both cases, the instrumental MET is dominated by mismeasurements of the hadronic system.
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This note is organized as follows.
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In Sections~\ref{sec:datasets} and ~\ref{sec:trigSel} we start by describing
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the triggers and datasets used, followed by the detailed object definitions (electrons, muons, photons,
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jets, MET) and event selection which is described in Section~\ref{sec:eventSelection}.
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We define a preselection and compare data vs. MC yields passing this preselection in Section~\ref{sec:yields}.
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We then define the signal regions and show the number of observed events and MC expected yields in Section~\ref{sec:sigregion}.
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Section~\ref{sec:templates} then introduces the MET template method and discusses its derivation
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in some detail, followed by a demonstration in Section~\ref{sec:mc} that the method works in Monte Carlo.
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Section~\ref{sec:topbkg} introduces the top background estimate based on opposite flavor subtraction, and contributions from other backgrounds are discussed in Section~\ref{sec:othBG}.
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Section~\ref{sec:results} shows the results for applying these methods in data.
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We analyze the systematic uncertainties in the background prediction in Section~\ref{sec:systematics}
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and proceed to calculate an upper limit on the non SM contributions to our signal regions in Section~\ref{sec:upperlimit}. In Section~\ref{sec:models} we calculate upper limits on the quantity $\sigma \times BF \times A$, assuming
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efficiencies and uncertainties from sample benchmark SUSY processes. We conclude in Section~\ref{sec:conclusion}. |
